Ink jet printers are non-contact printers in which droplets of ink are ejected from one or more nozzles in a print head. The ejected droplets are projected onto a substrate moving relative to the print head and progressively build up a printed image on the substrate from the dots of ink applied to the substrate. For convenience, the present invention will be described in terms of a substrate which moves with respect to a stationary print head. However, the invention may also be applied to printers in which the print head moves with respect to a stationary substrate. The invention can thus be applied to printers in which the substrate and the print head move relative to one another.
One form of ink jet printer comprises a source of ink under pressure, typically a reservoir or bottle of ink which is pressurised to from 0.1 to 3 bar, notably about 1 bar. The pressure is created, for example, by pressurising the air space above the ink in the bottle or reservoir. The ink is fed to a nozzle orifice in a print head through which it is ejected as a series of droplets onto the surface of the substrate. The print head typically comprises a row of such nozzle orifices arranged transversely to the direction of travel of the substrate. The flow of ink through each nozzle orifice is controlled by a solenoid valve. Typically, such a valve comprises an electromagnetic plunger journalled for axial movement within an axially extending electric coil. The distal end of the plunger is located within a valve head chamber through which ink flows from the reservoir to the nozzle orifice. When current is fed through the coil, this generates a magnetic field which acts on the plunger to move it axially and thus open, or shut, the inlet to a bore or conduit to the nozzle orifice. Typically, the magnetic field acts to retract the plunger against the bias of a coil spring to create a flow path between the valve head chamber and the nozzle orifice. When the electric current no longer flows in the coil, the magnetic field ceases and the plunger returns under the bias of the spring to seat against sealing ribs, lips or other means located at or around the inlet to the bore or conduit to the nozzle orifice so as to close the flow path to the nozzle orifice. Typically, a plurality of nozzle orifices are formed as a row in a plate, the nozzle plate, and each nozzle orifice is served by a bore through the plate connected to a solenoid valve. Typically, the nozzle orifice is provided as the distal end of a bore in a jewel nozzle, which is seated into the outlet end of a bore through the nozzle plate. Droplets of ink are ejected independently from one or more of the nozzle orifices upon actuation of the valve controlling the flow of ink to that nozzle orifice.
The valves are fed with ink from the reservoir via a manifold which serves to split and even the ink flow to each of the valves. The row of nozzle orifices is typically aligned transversely to the direction of travel of the substrate, so that simultaneous operation of the valves will cause a row of ink dots to be printed on the substrate. The valves are operated so as to deposit dots of ink upon the substrate at the desired locations on the substrate to build up the elements of a five, seven, eight or more dot raster image on the substrate. By suitable sequencing of the opening of the various valves serving the nozzles, dots of visible ink can be applied to form an alphanumeric or other image, a production date, product batch code, logo, bar code or other visible image on the substrate. Alternatively, the ink can be one which forms a UV fluorescent or magnetically readable machine readable image which is invisible to the human eye, for example a security or product authentification code. If desired, several nozzle plates can be combined in an array so as to print a wider image on the substrate and/or to achieve close printing of the dots on the substrate by staggering the nozzle plates with respect to one another.
For convenience, the term drop on demand printer will be used to denote in general such types of ink jet printer; the term ink will be used to denote pigmented and dye based inks which form a visible image and other fluid compositions which may not be visible but which are machine reader detectable; the term nozzle orifice will be used to denote the aperture through which the ink droplet is ejected; the term nozzle bore will be used to denote the bore connecting the valve head chamber with the nozzle orifice; and the term print head will be used to denote an assembly having one or more nozzle orifices and associated valves.
For a given nozzle orifice diameter, the size of the printed dot can readily be altered by varying the duration for which the valve is held open, and hence the amount of ink that is allowed to flow through the nozzle orifice for each opening of the valve. The form of the image which is printed can readily be altered by varying the sequence of operation of the valves in the print head so that droplets are ejected from the appropriate nozzles in the appropriate sequence to form the desired image. Such alterations of the images and the dot sizes can readily be controlled by a computer or microprocessor operating under an appropriate program or operating system.
Such drop on demand printers are widely available commercially and find widespread use in printing a wide range of both visible and non-visible machine-readable images on a wide range of substrates. Since the ink is ejected from the nozzle orifice at a pressure of from 1 to 5 Bar, the print heads can project the droplets from the nozzle orifices for a flight path length of 10 to 25 mms or more. This allows the print head to be located adjacent to a line of travelling articles in a production or packaging line, without the print head fouling the articles.
However, as the relative speed of travel between the print head and the substrate increases, a point is reached at which the valve cannot be operated at sufficient speed to eject droplets at sufficient frequency to form the desired image without creating some distortion. Typically, the limit for the speed of operation of solenoid valves in current use in an ink jet printer head is less that 1000 Hz. With increasing pressure on manufacturers to increase through put from a given production or packaging line, there is a growing demand to be able to print the dots onto the substrate at rates greater than this.
In an alternative form of ink jet printer, known as an impulse jet printer, a piezoelectric crystal or other transducer is applied to or forms part of a wall of an ink jet chamber having an ink inlet and having an ink outlet nozzle bore to a nozzle orifice. When a voltage is applied to the transducer, the transducer expands or flexes and causes a change in the volume of the ink jet chamber. This causes a droplet of ink to be ejected from the chamber and to exit through the nozzle orifice via the nozzle bore. The transducer can be caused to flex at very high frequencies by electronic control of the frequency of the electrical pulses applied to the transducer, so that such a print head can apply dots at frequencies up to 15 kHz or more. However, the volume of ink ejected through the nozzle orifice is dependent upon the extent of flexing of the transducer. This can be varied by varying the amplitude of the electric pulse applied to the transducer. However, each type of transducer operates consistently only within a narrow percentage, typically plus or minus 10%, of the optimum operating pulse amplitude, so that only a limited range of dot sizes can be achieved with a commercially available impulse jet printer. In order to increase the printed dot size it is necessary to print successive dots overlapping with one another. This limits the number of applications a given impulse jet head can be used for. Furthermore, such impulse jet heads project the droplet for only a small flight path distance, typically no more than 6 mms, since the pressure generated to expel the droplet of ink from the nozzle orifice is small. The pressure cannot be increased beyond a low threshold level, since the ink is held within the nozzle bore by the surface tension forces at the meniscus of the ink at the nozzle orifice. Also, the nozzle bore and nozzle orifice are formed in a thin wall of the chamber and the nozzle bore typically has a length to diameter ratio of less than 0.5:1. As a result the directionality of the flight path of the ejected droplets is reduced as compared to a drop on demand printer, where the nozzle bore in a jewel nozzle typically has a length to diameter ratio of 10:1 or more. Scatter of the droplets on the substrate increases if the flight path of the droplets is larger than about 2 mms for an impulse jet printer. This further limits the application of such impulse jet printers.
In a further type of ink jet printer known as a continuous ink jet printer, a charged jet of ink is ejected from a nozzle. This jet is broken up into individual charged droplets which are then steered towards the desired location on a substrate by applying varying electric voltages to deflection electrodes located adjacent the flight path of the individual charged droplets. Whilst such a printer can form droplets at high frequencies and can project the individual droplets for distances of 5 to 15 cms, such printers are complex and costly to construct and operate.
International Patent Application No PCT/SE97/01007 describes a solenoid type valve for a drop on demand ink jet printer which is claimed to be capable of operating at frequencies of up to 3 kHz. Such a valve incorporates a novel form of construction which enables the plunger to accelerate and decelerate rapidly at each extreme of its travel within the coil. To achieve this, the plunger is formed from two components, one made from an electromagnetic material so that it can be caused to move by the magnetic field generated by the coil, and a second lightweight plastic component for the distal end of the plunger. Such construction is complex and expensive. Furthermore, we have found that a drop on demand print head incorporating such a valve design does not print acceptable images. For example, at high frequencies of operation of the valve, the printed dots are uneven and there are many small satellite dots around each of the primary dots printed by the print head.
There thus still exists a need for a print head which is simple and yet can operate consistently at dot generation frequencies in excess of 1 kHz to generate uniformly sized droplets over a wide range of printed dot sizes using a wide range of inks or other fluids.
We have now devised a form of solenoid valve which can be operated at speeds of up to 8 kHz or more and yet print uniformly sized droplets over a wide range of dot sizes and operating frequencies. This is within the operating range of a conventional continuous jet printer. The valve of the invention thus enables a drop on demand ink jet printer to print images on a fast moving substrate where the use of an impulse jet or continuous jet printer had hitherto been consider the only viable form of printing technology available, and with reduced distortion of the printed image as compared to a conventional drop on demand printer. Such a valve can be made very compact, so that an array of the valves can serve one or more rows of nozzle bores in a nozzle plate to provide a compact print head with high print resolution.
A print head incorporating the valve of the invention can achieve the accuracy of placement of the printed dots and the long flight path of the droplets usually only achieved by slow speed drop on demand print heads. Surprisingly, the valve of the invention is capable of printing accurate droplets over a wide range of sizes and frequencies without significant satellite droplet formation. The valve of the invention can be produced to a consistent performance and the movement of the plunger within the coil can readily be controlled by regulating the frequency, amplitude, duration and shape of the current pulse applied to the coil using software to optimise print quality. This is to be contrasted with conventional drop on demand printers where the computer is used only to regulate the open time of the valve and to relate the timing and sequence of operation of the valve to the image which is to be produced.
Furthermore, the design of the valve of the invention readily lends itself to manufacture as an array of valves, each serving one of a plurality of nozzles in a single nozzle plate. This enables the nozzle plate and associated structure of the valve to be produced as a unitary construction with greater accuracy than where individual valves and jewel nozzles are used. A print head incorporating an array of nozzles and valves of the invention can be made to more consistent standards of performance than hitherto.
Furthermore, a print head incorporating the present invention can be made more compact than one using conventional valves. This enables small dots to be printed at closer spacing than could hitherto be achieved with a drop on demand printer.